The New World crocodile assemblage: crocodiles part VII – last in series!

Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

American crocodile, photographed in Mexico by Tomás Castelazo. Note paired holes that show where mandibular teeth have grown through the premaxillae! Image licensed under Creative Commons Attribution-Share Alike 2.5 Generic, 2.0 Generic and 1.0 Generic license.

A momentous thing is about to happen. Take a deep breath and prepare yourself. Today is the day when… I finish my series of articles on the crocodiles of the world. As you’ll recall if you read the previous six parts of this series (all of which are linked to below), I’ve managed so far to get through the Asian crocodiles, the Indo-Australian ones, and – most recently – the two distinct African lineages. That ‘only’ leaves us with the several New World crocodiles, and it’s this group of species – forming the so-called New World assemblage within Crocodylus – that we look at here.

We start with the best known member of this group: the American crocodile C. acutus. This is a mostly Central American species, well known for being tolerant of brackish and marine conditions. It’s a large to gigantic crocodile, the specific name referring to its proportionally long and slender snout. Actually, the name Crocodylus americanus Laurenti, 1768 might be a senior synonym of C. acutus Cuvier, 1807, but the former isn’t in use since the description it accompanies does not unambiguously pertain to this species. Having said that it’s “gigantic”, modern individuals tend to be between 2.5 and 4 m in total length, with lengths of 6 or 7 m being very much a thing of the past.

American crocodiles occur from coastal western Mexico all the way south to extreme northern Peru in the west and eastern Venezuela in the east. Its presence in southern Florida is well known, and in the Caribbean it occurs on Cuba, Jamaica, Hispaniola and Martinique (though, apparently, not elsewhere in the Lesser Antilles). Specimens have been reported from the Islas Marías in the Pacific (100 km off the coast of Mexico), the Caymans (370 km west of Cuba in the Caribbean), and archaeological remains show that it previously occurred on the Bahamas. Clearly, it’s able to make sea crossings and to live at sea for months at a time. Indeed, a specimen captured in 2008 off the coast of Mexico had stalked and non-stalked barnacles attached to its teeth and caudal scutes (Cupul-Magaña et al. 2011).

Adult American crocodiles have a distinctive hump along the midline of the snout, anterior to the eyes, and their irregular and slightly asymmetrical arrangement of dorsal armour is also distinctive. The dorsal osteoderm compliment of the American crocodile is strongly reduced compared with that of most other living crocodylians; it is in fact the most reduced osteoderm compliment of any living crocodylian. In some individuals, there are only four or even just two scutes arranged transversely across the middle part of the body (six or eight are more typical), and two transverse scute rows are typically absent between the dorsal shield and neck shield, creating an unarmoured band across the base of the neck (Ross & Mayer 1983). Crocodiles normally have 16 or 17 transverse osteoderm rows in the dorsal shield, but some American crocodiles only have 14 (Ross & Mayer 1983).

Dorsal scute patterns in (left to right): Cuban crocodile, Orinoco crocodile, and two American crocodiles. Note the substantially reduced scute compliments in the American crocs. Image from Ross & Mayer (1983).

This reduced armour compliment is very likely related to the American crocodile’s adaptation to life in brackish and marine environment; it probably isn’t coincidental that the other crocodile with a reduced armour compliment is the Saltwater crocodile.

Crocodylians and seed dispersal, again

American crocodiles have catholic diets, with fish dominating their diets but birds and mammals being consumed too. Juveniles eat insects, frogs, small turtles as well as fish and small mammals. Intriguingly, Casas-Andreu & Quiroz (2003) found a large number of plant seeds inside the American crocodile droppings they analysed from Mexico. They assumed that these seeds originated from the stomach and gut contents of animals that the crocodiles had eaten, and noted that crocodiles might play a hitherto overlooked, but important, role in seed dispersal.

While this all sounds likely and sensible, long-time readers with good memories might recall that occasional fruit-eating and even leaf-eating is now known for several crocodilian species: I covered this subject back in 2008; more recently, another study has reported seed dispersal and possible fruit consumption in American alligators Alligator mississippiensis (Platt et al. 2013). Could it be, then, that at least some of those seeds in the Mexican droppings represent primary acts of plant consumption on the part of the crocodiles, not secondary acts resulting from the ingestion of herbivores?

Placing C. acutus in the crocodile tree – with complications from non-monophyly

Views on how the American crocodile might be related to other crocodiles have varied quite a bit. It’s often been considered close to the Cuban crocodile C. rhombifer, Mexican crocodile C. moreletii and Orinoco crocodile C. intermedius within the ‘New World assemblage’, but some studies have – surprisingly – recovered C. siamensis and C. porosus as part of the same clade as these American species (Densmore & Owen 1989).

The hypothesis that there might be a ‘New World assemblage’ consisting only of the American crocodile, Cuban crocodile, Morelet’s crocodile and Orinoco crocodile (Brochu 2000, Oaks 2011) seems logical on biogeographical grounds (but, ha, that doesn’t mean it’s true). Several morphological characters seem to support the monophyly of the New World assemblage, most notably the presence of a median boss on the snout formed from slightly elevated nasal bones (Brochu 2000) and a reduced post-occipital scute row where the scutes are small and low in number (this latter feature has evolved convergently in some other crocodiles, most notably the Siamese and Saltwater crocodiles).

Man et al. (2011) found a strong relationship between the American crocodile and Nile crocodile in their analysis of mitochondrial genes. However, they didn’t include any other New World species, so it mustn’t be assumed that they necessarily demonstrated an exclusive relationship between these two species. Their findings do, however, add support to the view that the New World assemblage is closer to the Nile crocodile than to members of the other living crocodile lineages. Oaks (2011) supported monophyly of a New World assemblage, with the Nile crocodile and Sacred crocodile being successive outgroups to this clade (see the previous articles in this series – links below – for more on those species).

The distribution of the American crocodile is interesting in that it occurs on both the Caribbean and Pacific sides of Central America. It’s plausible that members of the respective populations might have moved across land to get to the other side of the landbridge. But it’s also plausible that the populations have been separate for a very long period of time. Given that the Panamanian Isthmus formed more than two million years ago, the potential for reproductive isolation here very much suggests that cryptic lineages await recognition. Fossils show that C. acutus was present in Costa Rica at least about two million years ago (Mead et al. 2006), supporting the view of a lengthy and perhaps complex history in the region.

Range map for American crocodile by Brandon Sideleau, (c) used with permission. Dark green is present/likely present, light green is unknown or extremely patchy presence, orange is extirpated.

There is in fact already some evidence that C. acutus as conventionally recognised is not monophyletic (though, don’t get me wrong: populations don’t have to be monophyletic in order to be considered a ‘species’. A ‘species’ is, after all, pretty much whatever the hell we want it to be). Milián-García et al. (2011) found Cuban C. acutus populations to form a clade with the Cuban crocodile C. rhombifer, rather than with C. acutus populations from Central America. This is at odds with what we’d predict based on morphology, since the Cuban crocodile is extremely distinctive in appearance while the Central American and Cuban C. acutus populations are highly similar. One possible solution might be to sink the Cuban crocodile into C. acutus (perhaps down-grading it to ‘subspecies’ level); another might be to place the Cuban C. acutus population within C. rhombifer. A third possibility is that the Cuban C. acutus population actually represents an additional species that needs naming: it looks like C. acutus from Central America, but the similarity is symplesiomorphic and the two have in fact been separate for 2 or more million years (Milián-García et al. 2011). More work is needed to resolve this issue.

Captive Cuban crocodile, photo by Darren Naish.

On Cuba, American crocodiles live in sympatry with Cuban crocodiles. Both are known to hybridise in captivity, and genetic evidence from wild animals seems to confirm that morphological ‘mosaics’ involving the characters of both species are indeed naturally occurring hybrids (Milián-García et al. 2011). Cuban crocodiles declined substantially during the 19th and 20th century, largely due to hunting pressures, and today they’re entirely restricted to the Zapata Swamp in western Cuba.

Unfortunately, illegal hunting of the species continues, as does modification of the Zapata Swamp region. Hybridisation with the American crocodile may represent a new and unanticipated problem for the persistence of the Cuban crocodile and it has been recommended that future management plans aim to reduce or prevent interspecific hybridisation occurring (Milián-García et al. 2011). As is clear from the accompanying photos, the Cuban crocodile is pretty awesome in appearance. It has especially prominent squamosal horns, an especially rugose-looking integument, and a neat, mottled pattern that involves black blotches on a greenish or yellowish ground colour. Some sources say that it has proportionally longer legs than other crocodiles: does anyone know if this has been confirmed?

One of the world’s most poorly known crocodiles is Morelet’s crocodile C. moreletii, named in 1851 for specimens collected by French naturalist Pierre Morelet in Lake Petén Itzá (then known as Lac Florès), Guatemala. The names Central American crocodile and Mexican crocodile are sometimes used for this species. I personally prefer these to ‘Morelet’s crocodile’ since they at least remind you that the species is an American one, but I’m also a stickler for history and don’t wish to denigrate Morelet’s role in its discovery. The species has a confused early history, since at least one of the first specimens was shipped to Cuba and mixed with a consignment of Cuban molluscs. This meant that the crocodile was then assumed to be a Cuban crocodile, and distinct species status for C. moreletii wasn’t really accepted until 1924.

Morelet's crocodile: image by Sheri L. Williamson, used with permission. Finding good photos of this species is not easy!

Morelet’s crocodile is usually dark brown or blackish with a snout of ‘generalised’ form (that is, neither especially robust nor especially elongate). In keeping with this anatomy, it seems to be a dietary generalist, with prey items including snails, turtles, mammals and fish. Platt et al. (2007) reported cases in which Morelet’s crocodiles scavenged on dead cattle and also described an incident where a Morelet’s crocodile found and partially consumed a tapir killed by a Jaguar Panthera onca.

While often stated to be strictly restricted to freshwater lakes, rivers and pools in forests and savannahs, Morelet’s crocodile also inhabits brackish lagoons in parts of its range. It recalls the American crocodile in having a reduced and somewhat asymmetrical dorsal osteoderm compliment: just three or four osteoderms might be present in each transverse row in the dorsal shield, and detached lateral scutes are always present on the flanks. At least some of the four to six scutes present in the cervical row closest to the head are larger than those of the American crocodile (Ross & Mayer 1983).

Another one of those art-meme things. The cover photo of Guggisberg's Crocodiles proved inspirational to several artists over the years: here, Graeme Sims (above) and Alan Male.

Morelet’s crocodile is generally stated to be a small species that might not usually exceed 2.4 m. However, individuals of over 3 m and even 4.1 m were observed in the 1970s and 90s (Pérez et al. 1991) and there are suggestions that animals about 5 m long might previously have existed. This is clearly not a giant species, but the fact remains that it probably can reach a greater size than usually thought, and it may owe its small modern average size to decades of intensive and selective hunting (Ludwig & Ralf 2006). The skin of this species has been used as a high quality leather and exploitation for the leather trade resulted in its local extinction across part of its range. Legal protection was granted in 1981 and the species has partly recovered since then. John Platt wrote about the conservation status of the species over at Extinction Countdown back in April 2011.

Morelet’s crocodile is known to hybridise with C. acutus on the Mexican Yucatan Peninsula (Cedeño-Vázquez et al. 2008).

The Orinoco crocodile C. intermedius is a critically endangered member of the New World assemblage found only in Colombia and Venezuela [adjacent image by Greg Hume]. Its long, slender and often gently upcurved rostrum is usually regarded as distinctive, but it can sometimes look extremely similar to the American crocodile, so much so that there are cases where captive specimens identified as Orinoco crocodiles were actually misidentified members of C. acutus (Trutnau & Sommerlad 2006). The two can, however, be distinguished thanks to the convex area present on the midline of the rostrum in C. acutus (no such structure in C. intermedius), the longer mandibular symphysis of C. intermedius, and the less reduced dorsal osteoderm compliment of C. intermedius.

Like the American crocodile, the Orinoco crocodile is supposed to reach gigantic size – that is, there are dubious accounts of specimens 7 or 8 m long. 5.2 m is the more accepted maximum recorded length, though even this is exceptional compared to modern animals, adult males averaging just over 4 m. Orinoco crocodiles vary in colouration from greyish to brownish or yellowish, and dark spots and blotches are often present over the dorsal surface. Black specimens are on record.

The Orinoco crocodile seems to be especially close to the American crocodile (Brochu 2000, Oaks 2011).

And it’s at this point that I have to stop – since now, finally, I’ve completed my planned species-by-species review of the living crocodiles. Yay! Basically, this series of articles should be seen as a sort of review of ideas about the phylogenetic hypotheses and species-level diversity of extant crocs. There is, needless to say, tons on the ecology, behaviour, biology and conservation of these species that I haven’t covered. We will, of course, be coming back to crocodylians (and other crocodyliforms, and other crocodylomorphs) on numerous occasions in the near and far future. Until then…

Human hand and Crocodylus hand print. Here I should say something poetic about hoping that members of these two lineages continue to persist side by side... but that would be pretentious. Image by Darren Naish.

About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The distribution map for the American Crocodile seems to be missing Martinique. It seems to be rather odd for the species to be present there but not anywhere else in the Lesser Antilles nor in Puerto Rico. How likely is it that resident populations throughout the Caribbean were extirpated in (pre-)historic times?
It also seems quite fitting and ominous that the last wild Cuban Crocodiles are restricted to the Zapata Swamp that also was the last haunt of the extinct Cuban Macaw.

American crocodile lives in rivers in many parts of its range, and there are places (i. e. in Panama) where crossing from Caribbean watershed to Pacific watershed would require a crawl of less than a mile.

What is more interesting is that Morelet’s crocodile has never been able to cross the continental divide, although now there are small populations on the Pacific side of Chiapas founded by croc farm escapees.

The fact that American crocodiles once inhabited the Bahamas still amazes me. Though i thought it wasthe Cuban species that once lived here. I wonder why it became extinct?

Crocodilian behavior and ecologyis fascinating. Especially crocodilian socialbehavior, which seems understudied or at least underrepresented in the online literature. The Cuban crocodile seems especially social–cooperative hunting behavior was reported for some captive individuals. I’m sure there is much more to learn.

The problem with any range map is obviously data, or lack thereof. Martinique for example may perhaps still have C. acutus but if there hasn’t been a confirmed sighting in many years we classify them as itinerants for the purposes of the map. The one linked above includes the likely itinerant range.

That said, if there’s good evidence to support changes then we’d be happy to amend it.

Awesome article and congrats on finishing the series on Crocodylus! Two things:
1) Crocodile skulls? (You know, just to wrap it up)
2) Not sure of the exact issue, but I do have a NatGeo article that shows a Cuban croc skull found in a Bahamas sinkhole (recognizable by the squamosal horns), so does that suggest both acutus and rhombifer were once extant in the Bahamas?

C. rhombifer is regarded as previously extant in Bahamas (and Cayman Islands) based on fossil evidence, probably the skull you’re referring to. I believe there’s also fossil evidence of C. acutus in Bahamas though I can’t recall the source, it’s certainly well within the potential itinerant range for the species even today. As populations grow, itinerant sightings become increasingly common and the distribution range tends towards pre-hunting / historical range, habitat change notwithstanding (which is of course the most likely thing that’s going to affect any modern range post-recovery). Sadly I can’t see C. rhombifer expanding its range much beyond its current distribution anytime in the foreseeable future.

Jerzy:
I’d guess it’s the combination of very old individuals and tall tales, or optimistic estimation of the size for record animals. Hunters exaggerating the awesomeness of their catch, etc. Re old crocs: wouldn’t increase of human pressure on their habitat decrease life expectancy of the animals? The bigger the crocodile, the more food it needs, and the harder it is to subsist on a territory hemmed in by human activities, or impoverished by pollution, overfishing, hunting of local fauna for food and pet trade, etc.

The distance between The Bahamas and Cuba is not that great even today. THe islands closest to Cuba are quite dry though, so any crocodile that made it to those islands today would have to be tolerant of marine conditions as freshwater bodies of any great size are lacking. Also, terrestrial preyis rather thin on the ground and this probably always has been true. In short, given the right opportunities, C. acutus at least probablycould make it to the Bahamas–but populations would be unlikely to persist at hight densities and individuals would would probably remain small.

Wow, the teeth on both some living and old Cuban crocs (like that skull from the Bahamas) are really impressive. I really reading a long time ago that Cuban crocs might be “specialised for” predation on small ground sloths. I wonder where that idea came from – I mean, was it inspired by the discovery of associated specimens or preserved evidence of interaction or something?

“This reduced armour compliment is very likely related to the American crocodile’s adaptation to life in brackish and marine environment; it probably isn’t coincidental that the other crocodile with a reduced armour compliment is the Saltwater crocodile.”
An interesting correlation to be sure, but what might be the functional explanation?

“populations don’t have to be monophyletic in order to be considered a ‘species’. A ‘species’ is, after all, pretty much whatever the hell we want it to be”
Point taken, but surely monophyly is something “we” want in “our” species!

On the reduced dorsal scute compliment of American and Saltwater crocs, ChasCPeterson (comment # 22) asks “An interesting correlation to be sure, but what might be the functional explanation?”. The implication is that these quasi-marine species have evolved under selection related to the making of sea crossings, with their reduced scute compliment lightening their bodies and hence making them more buoyant than their relatives. As usual with hypotheses of this sort, I don’t think that this idea has been tested. Note that lightened and reduced scute compliments are present in other crocodyliforms strongly adapted for aquatic life (Deinosuchus, thalattosuchians).

As for ” surely monophyly is something “we” want in “our” species!”… yikes, can of worms! Surely some (many?) ‘species’ are monophyletic bits of lineages, but how many are not? I mean: we know that some species incorporate genes from more than one ancestor species (example: a familiar bipedal primate), some really are bastard, hybrid offspring of phylogenetically disparate parent species (example: Père David’s deer, parthenogenetic whiptail teiids), and others are paraphyletic parts of branches that don’t include all of their lineages (example: brown bears in some phylogenies, Mountain hare). And I’m not going to mention non-tetrapods… especially not non-animals.

@Darren: not to mention the problem of insular species. If their mainland ancestors are still extant, it’s quite unusual for the colonizing population to not be deeply nested in their mainland ancestors.

And not to mention dogs, pigs, cattle…

Even humans could be paraphyletic, if all of those alien abductions have been used to establish breeding populations of humans on other planets, and they’ve speciated in their new habitats. How would we even know, until the aliens revealed their work? Monophyly is an assumption, although admittedly it’s a pretty robust assumption in most cases.

Thanks for the response–good points about monophyletic (or not) species.
I’m not buying the buoyancy argument though. Tetrapod buoyancy comes almost entirely from lung air except in the very fattest types, and as far as I know, all crocs can float just fine with whatever complement of dermal armor they’ve got. And salt water just makes everything more buoyant.

@Jerzy You can count density growth rings in bone, but it’s far from reliable; there are too many variables that can influence changes in growth rate that affect ring visibility, not to mention resorption can confuse matters further.

@ChasCPeterson I agree, it doesn’t make sense for buoyancy to be a significant factor in the amount of dermal armour for the exact reason you state. It makes more sense in fact that extant species that spend more time wandering around on land have developed even more robust osteoderms for protection next to their more aquatic brethren.

@naishd C. rhombifer definitely has more robust / longer legs than most other species. I can’t say if it takes the crown because I don’t think anyone has done a thorough comparison, though that would be interesting when linked to behaviour. Anecdotally though, anyone who has experienced Cuban crocs would support the idea!

@ChasCPeterson
Perhaps it’s not buoyancy that’s being increased by reduced dermal scutes but flexibility. Note that in the most specialized marine crocodiliformes known dermal scutes were almost entirely absent.
@Darren: Congrats for finishing the series. Any chance of a series on the world’s alligatorids?

Chas:
“I’m not buying the buoyancy argument though. Tetrapod buoyancy comes almost entirely from lung air except in the very fattest types, and as far as I know, all crocs can float just fine with whatever complement of dermal armor they’ve got. And salt water just makes everything more buoyant.”

I agree. The floating hypothesis doesn’t really hold water (if you pardon the expression).

Why would marine crocodylians need to be more flexible? For vertebrates that spend significant amounts of time swimming in open water, rigidity – not flexibility – of the body would seem to be a more useful trait, as a rigid body makes it easier to attain high swimming speeds.

@ Dartian: But isn’t a more flexible body, and particularly a flexible skin, consistent with the lifestyle of an animal that depends on fish for a large part of its diet? I can’t help thinking that other fish predators (otters, seals, sharks, dolphins, ichtyosaurs…) are agile swimmers with no skin armor.

irenedelse:
“But isn’t a more flexible body, and particularly a flexible skin, consistent with the lifestyle of an animal that depends on fish for a large part of its diet? I can’t help thinking that other fish predators (otters, seals, sharks, dolphins, ichtyosaurs…) are agile swimmers with no skin armor.”

Crocodylians (extant crocodylians, anyway) don’t chase after fish or other prey in the open water, at least not over any significant distances. They are ambush hunters, and can therefore not really be compared to those pursuit predators that you listed. (Do extant ocean-going crocodiles even feed while they’re out at sea?)

If there is any selection in extant ocean-going crocodiles for increasing swimming performance, it’s more likely to be related to the need to cross those open expanses of sea as quickly as possible – not for hunting.

Excellent work on Crocodylus. But you might as well go ahead and finish the whole mess. There aren’t that many species of crocodylians (extant ones, I mean) outside that genus.

Speaking of species, paraphyletic species are unavoidable given some species concepts, including the popular “biological” species. Interbreeding is, after all, plesiomorphic. If we have three allopatric populations, A, B, and C, of one species, with the relationship (A(BC)), and B evolves reproductive isolation from the other two, the remaining group AC is paraphyletic. Of course if there’s gene flow between A and C the paraphyly will eventually be repaired, or at least become invisible.

Hmm, weighing in on buoyancy idea, turtles are pretty much THE armoured marine vertebrates. They seem to cope fairly well (albeit are not fast swimming predators). Indeed, they rely on being negatively buoyant to sleep on the sea bed.

I’m now mildly curious as to density of a turtle, lungs full (of air) versus lungs empty.

I don’t know how salt water crocodiles sleep at sea (or even if they do), whether they sleep at the surface like dolphins or resting on the sea bed like a turtle.

Sea turtles and armour… remember that dermochelyids (leatherbacks) and protostegids (Archelon and kin) have strongly reduced shells (some people even say that the leatherback has no shell at all), and these are the most pelagic, most marine of all turtles. I’m really not sure about the buoyancy hypothesis, but I wonder if there might be something in it.

I’d also point out that out at sea, a croc is probably most at risk from a large demersal predator striking out of the blue from below, whereas in the more “normal” crocodile shallow aquatic environment the only threat axis would come to the topside (even another croc trying to lever up and bite).

In short, at sea, if the armour is to be of effect, it would need to encase the animal, while on land/shallow water, top armour is adequate. At that point, presumably its is “easier” for the armour to be reduced and subsequently lost altogether, along with the associated cost of its production, rather than grow an enormous amount of extra armour. This would be a shorter transition, with progressive gains in fitness along the way as compared to the long and costly process of increasing armour, with little gained until a very large portion of the body is armoured. (Even then, armoured to such an extent that it actually improves the protection of a large, powerful croc to the point that its retention is of selective advantage compared with losing it. There would be a survival advantage to the juvenile, but for a r selected species I guess there’s more benefit in rapid maturation.)

It’s hard to find specific data on the sleep behaviour of leatherbacks… most statements about sleep in sea turtles are generic ones that apply to all species (and that’s confusing, since there are some species where animals sleep on the seafloor, sleep while suspended in the water column, or even haul out and sleep on land). This article by Anna Goode says that leatherbacks sleep while floating at the surface.

Well, I’m tempted to make the suggestion that perhaps salt-water tolerant crocodiles which exhibit scute reduction are in fact adapting to sleep at the surface, and by extension perform long pelagic forays. Kind of suggests that they feed at sea.

I would imagine that there must have been some attempt to satellite tag modern saltwater crocodiles, after all it is a highly charismatic animal, with enormous potential for sponsorship by (shudder) the Discovery channel. Then again, how much has saltwater crocodile behaviour been perturbed by man..

All the sea-going croc stuff, though, is referring to crocodiles moving around coastlines. They do strike out to sea occasionally (which explains why they turn up occasionally on offshore islands, and I’ve heard at least one story of a saltwater crocodile approaching an oil rig that was quite some distance off shore) but that is rare behaviour and unlikely to be one of the crocs you have a satellite transmitter on. I’ve got some data on crocs swimming to an offshore island (where we tagged them), and while it wasn’t an enormous distance back to the mainland, they swam in every case in a direct line to the nearest estuary mouth. They have quite excellent navigation abilities. Writing this up now.

Based on the evidence it appears that sea-going crocs are basically out there to get from A to B, although there’s no reason why they couldn’t take prey opportunistically if they encountered it. There are also cases where people have been attacked some distance off the coast by crocodiles, although it’s hard to know if that was predation or simply (and more likely) defensive behaviour.

Hmm, third attempt to post this comment. Apparently crocodiles have been known to attack and feed on dugongs at sea, although this is hardly a pelagic behaviour. Will attach link in a following post (in case this is what has been killing my posts).

In addition to flotation, I’d pitch in the possibilities of streamlining and marine fouling. Off-hand, I don’t know of a lot of marine fish that have scales the size of scutes, and it may be that scute reduction makes swimming more efficient. That’s something that could actually be tested pretty easily using young crocs in a flow tank.

The other thought is that, since crocs definitely can provide substrate for barnacles and similar things, all of which would “foul” them and slow them down in the water, it’s possible that scute reduction makes it somewhat harder for parasites to get attached, thereby making it easier for them to swim long distances. This one’s harder to test, but I suppose you could get barnacle larvae, put them in a saltwater tank with various crocs, and measure attachment rates and survivorship on the barnacles.

Last winter I was on Maui and saw a green turtle laying on the beach, sleeping. There was WHAT APPEARED TO BE (still not positive) a barnacle on the left side of its face, and the skin below was red and puffy. The first thing I thought was “can anyone get this barnacle off?” The second thing was “how did this barnacle get here?”

“I’m now mildly curious as to density of a turtle, lungs full (of air) versus lungs empty.”

Not much data. I studied two small freshwater species (Herpetologica 64:141-148, 2008), and the highest and lowest specific gravities we measured in unmanipulated turtles were 1.10 (Sternotherus) and 0.95 (Chrysemys). I don;t claim these are limits, because they could compensate for added weights or floats equivalent to up to 6% of body mass in air. One stinkpot was unable to surface to breathe when a weight brought its SG up to 1.19, so the natural maximum is presumably lower than that.

John Harshman: “Interbreeding is, after all, plesiomorphic.”
My preferred form of the slogan is “Interfertility is symplesiomorphy” (still only three Google hits, two on Tet Zoo).
The tooth-pierced premaxilla shown in the top photo is a pretty common feature in C. porosus also, going by the skulls I’ve seen. Also, it reminds me of a condition that has evolved in at least two elapid snake lineages, Death Adders Acanthophis and Taipans Oxyuranus, where the fangs are so long that they invariably pierce the intramandibular tissue. Not only can they bite you with their mouth closed (if so inclined), but every individual I’ve ever checked has fresh punctures and/or old scars on the infralabial scales. The maxillae are fairly short and (hence, in part) relatively highly mobile in these groups, but not to anything like the degree in solenoglyphs.

Does anyone else love the crocs of time montage Darren drew a while ago? If so, let’s try to buy an episode of Tetrapodcats and make him discuss the species depicted in his own imitable style. We could donate with the note “For the croc episode” in the paypal description. I reckon (this is based on guesses derived from dropped comments in the podcasts) if could summon up ~£100-200 we may be able to pull it off. I would appreciate it if Darren could (if he likes the idea) mention this in other formats (Twitter, TZP etc.) to aid in the accumulation of cash, and correct the estimate of cost. Best wishes.

Oh, and @ChasCPeterson, thanks awesome! When you say they could compensate, does this mean they could (somehow) rest of the bottom when their apparent density was <1?

“When you say they could compensate, does this mean they could (somehow) rest of the bottom when their apparent density was <1?”
Not quite–if you attach a float that experimentally brings their density to <1, they can compensate (within limits) by chronically retaining less lung air to re-achieve their desired buoyancy (and conversely, will retain more air to compensate for an added weight).
Turtles have excellent buoyancy control. There's good evidence that sea turtles will adjust their lung contents before a dive in such a way as to achieve neutral buoyancy at the depth they plan to dive to!
Also, through watching my captives in a 1-m-deep aquarium, Ive become convinced that they can control pitch and probably roll by moving air around inside their lungs (presumably using smooth muscle; we know it's there), but I haven't figured out a way to prove it experimentally.

@ChasCPeterson: I’m tempted to agree that attitude control is possible for turtles through some sort of internal process. I’ve watched green turtles in the wild both pitch and roll without appearing to even twitch a muscle. Previously I just assumed that they were so efficient that I simply couldn’t make out the small movements they were making in front of me, but your idea is so very appealing. Plus it would be very cool were it to be proved (somehow). Best of luck.

I suspect this is the most likely answer. Which means the only way to test this is to better understand what crocs use their osteoderms for.

I’m surprised no one mentioned the fact that the two crocs with the most reduced body armour are also the largest extant crocodylians today. Similarly, leatherback sea turtles are the largest turtles today and also show completely reduced (basically gone) shell. Even your “standard” sea turtle has extensive shell reduction compared to something like a diamondback terrapin. Note also that the smallest crocodylians today (caimans, Osteolaemus and Alligator sinensis) are all heavily armoured.

In other words, maybe we see this reduction in armour because the animals are all reaching sizes large enough that their only real dangers are equally large conspecifics. Take away the predators and you might take away the selection pressure for armour.

Croc osteoderms have also been implicated in back bracing for terrestrial locomotion. A fully marine croc would obviously have no need for that ability, which could also free up selection pressure resulting in osteoderm loss.

ChasCPeterson wrote: Also, through watching my captives in a 1-m-deep aquarium, Ive become convinced that they can control pitch and probably roll by moving air around inside their lungs (presumably using smooth muscle; we know it’s there), but I haven’t figured out a way to prove it experimentally.

If you haven’t read it already I’d recommend reading Uriona and Farmer’s work on croc respiratory muscles and how they influence buoyancy.

Uriona, T.J., Farmer, C.G. 2008. Recruitment of the diaphragmaticus, ischiopubis and other respiratory muscles to control pitch and roll in the American alligator (Alligator mississippiensis). JEB Vol. 211:1141–1147.

Turtles have similar diaphragmatic muscles, so I wouldn’t be surprised if they used a similar technique.

“Some sources say that it [the Cuban croc] has proportionally longer legs than other crocodiles: does anyone know if this has been confirmed?”

I don’t know, but it does look rather leggy. If it’s any help, here’s a link to another picture I took at Skansen Aquarium, Stockholm. It shows the limbs proportions pretty well. This is, I think, the mate of the animal in the photo I sent for this article earlier.

@Jurassosaurus
It is rather surprising that no one seems to have considered that such large predators as the largest living crocodilians have a reduced need for body armor. How vulnerable do you think Mesozoic marine crocodilians were to shark attack?? And how often are leatherbacks predated upon by large sharks?

Jurassosaurus:
“maybe we see this reduction in armour because the animals are all reaching sizes large enough that their only real dangers are equally large conspecifics. Take away the predators and you might take away the selection pressure for armour.”

But that same applies to virtually all other large (i.e., species reaching adult lengths of more than circa 4 meters) extant crocodilians too. Adult Nile crocodiles, black caimans, gharials, etc., are also effectively immune to interspecific predation as adults – yet they haven’t reduced their body armour. Perhaps it is intraspecific aggression that is more important here? Do saltwater and American crocodiles typically live at significantly lower population densities than freshwater-living crocodilian species?

Dartian wrote: But that same applies to virtually all other large (i.e., species reaching adult lengths of more than circa 4 meters) extant crocodilians too. Adult Nile crocodiles, black caimans, gharials, etc., are also effectively immune to interspecific predation as adults – yet they haven’t reduced their body armour.

Not necessarily. Compared to smaller crocodylians, all the above examples (well, maybe not black caimans) do show reduced armor. As for why it is not more reduced, I suspect it has to do with the ecology of each species. Adult Nile crocs still occasionally fall victim to hippo and leopard attacks (the latter true for smaller adults). They also readily challenge lions for carcasses. Saltwater crocodiles are pretty much predator and competitor free once above a relatively small size threshold. American crocs seem to have a similar ecological relationship. Alligators live around pumas and bears. I’m not sure how often the three interact, but it might be enough (or in the past, have been enough) pressure to retain more armour than is seen in C.acutus and C. porosus.

As for leatherbacks, I’ve read some anecdotal accounts of shark/orca predation on them, but I haven’t come across any concrete data on estimated frequency.

Jurassosaurus:
“Saltwater crocodiles are pretty much predator and competitor free once above a relatively small size threshold. American crocs seem to have a similar ecological relationship.”

Saltwater crocodiles are sympatric with tigers in Asia and American crocodiles are sympatric with jaguars – and both tigers and jaguars are confirmed killers of fairly large crocodilians. As for other predators, great white sharks Carcharodon carcharias reportedly prey on adult C. acutus in Colombia. The source for that is Medem (1981); unfortunately, I haven’t seen this publication myself but it is cited by Somaweera et al. (2013).

Incidentally, Somaweera et al. (2013) – a source that I was unaware of at the time when I wrote my previous comment in this thread – say this, in passing (on page 40):
“[A] reduced armor may facilitate long-distance swimming in C. porosus”.

a condition that has evolved in at least two elapid snake lineages, Death Adders Acanthophis and Taipans Oxyuranus, where the fangs are so long that they invariably pierce the intramandibular tissue. Not only can they bite you with their mouth closed (if so inclined), but every individual I’ve ever checked has fresh punctures and/or old scars on the infralabial scales.